The invention relates to a circuit for adjusting a fixed amplitude ratio between a chrominance signal separated from a composite color television signal and a luminance signal by means of a variable-gain amplifier, the gain of which is controlled in dependence on the amplitude of a color synchronizing signal.
In a television receiver, the composite color television signal (CVBS), demodulated from the intermediate frequency, is split into the brightness signal and the chrominance signal which are further processed separately. The chrominance signal amplifier must be controlled automatically so as to cause the color information on the picture tube to be always in the correct ratio to the luminance signal and to make it independent of the tuning of the receiver or of the tolerances in the receiving circuit. As the chrominance signal-to-luminance signal ratio depends on the picture content, the setting of the chrominance signal amplifier is controlled in dependence on the amplifier of the color synchronizing signal (burst), as it is always in a defined ratio to the luminance signal. The required control range of the chrominance signal amplifier is determined by the tolerances of the total transmission path from picture recording to the color television receiver, the amplitude being very strongly affected by a slight detuning of the receiver, which is more specifically caused by the fact that the chrominance signal which is transmitted at the carrier frequency is, in the customary receiver designs, located at the band limit of the intermediate frequency filter. Allowing for a certain amount of overdrive, this results in a total required setting range of approximately 24 dB.
In the television receivers which have been customary so far, as described in, for example, the periodical "VALVO Mitteilungen" 63, April, the control of the chrominance signal amplifier is effected by deriving a voltage for controlling the amplifier from the amplitude of the color synchronizing signal at the output of the amplifier. This results in a closed control loop which keeps the value of the color synchronizing signal at the output of the amplifier as constant as possible. However, it is very difficult to optimize such a control and it has all the well-known problems of closed control loops.
The periodical "Elektronik", 1981, Volume 16, pages 27 to 35, inclusive, describes a television receiver design which operates digitally to a considerable extent. However, this periodical contains very little information about the setting of the chrominance signal amplifier, from which it has been found that also in this design a closed control loop is used for setting the chrominance signal amplifier.